1. Field of the Invention
This invention relates to a voltage to frequency converter, and more particularly to a voltage to frequency converter wherein an input voltage that is used in an electronic watt hour meter or the like is converted to a frequency corresponding to the input voltage.
2. Description of the Related Art
FIG. 7 shows a prior art example of a voltage to frequency converter that converts an input voltage to a frequency corresponding to the input voltage. FIG. 8 shows the waveforms of the various units of FIG. 7. In FIG. 7, there is provided a selector 8 that obtains a resultant current I3 of a reference current I2 and an input current I1 corresponding to an input voltage E. There is further provided an integrator 1 that obtains an integrated voltage A by integrating resultant current I3 of this selector 8. Next, there is provided a comparator 2 that compares integrated voltage A and a reference voltage AG and outputs low level when this integrated voltage A is higher than reference voltage AG and outputs high level when it is lower. There is further provided a selection signal generator 3 that has a JK flipflop which outputs a selection signal K when a pulse signal CLK changes from low level to high level. This selection signal K controls a switch SW that allows the passage of reference current I2 of selector 8 described above. There is also provided a pulse signal generator 5 for generating pulse signal CLK described above.
In such a circuit layout, when output signal K of JK flipflop 31 of selection signal generator 3 is high level at a time t0, switch SW is closed, and resultant current I3 flowing into capacitor C1 of integrator 1 becomes: EQU I3=I1-I2 (&lt;0), I2&lt;I2
and integrator 1 commences integration in the plus direction. When output voltage A of integrator 1 becomes higher than reference potential AG at a time t1, output signal J of comparator 2 becomes low level. When output signal J of comparator 2 becomes low level, the inputs of JK flipflop 31 become:
J: low level PA1 k: high level (=Q output) PA1 J: high level PA1 K: low level (=Q output)
with the result that output signal K of JK flipflop 31 becomes low level at the rising edge of pulse signal CLK at a time t2 as shown in FIG. 8.
When output signal K of JK flipflop 31 is low level, switch SW is opened, and resultant current I3 flowing into capacitor C1 becomes: EQU I3=I1 (&gt;0)
and integrator 1 commences integration in the minus direction. When output voltage A of integrator 1 becomes lower than reference voltage AG at a time t3, output signal J of comparator 2 becomes high level. The inputs of JK flipflop 31 becomes:
and output signal K of JK flipflop 31 becomes high level at the rising edge of pulse signal CLK at a time t4 as shown in FIG. 8. When output signal K of JK flipflop 31 is high level, switch SW closes, and returns to the initial condition.
Thus when switch SW1 is closed, a quantity of electric charge Q1 stored in capacitor C1 of integrator 1 is: EQU Q1=(I2-I1).times.T1
In contrast, when switch SW is open, a quantity of electric charge Q2 discharged from capacitor C1 of integrator 1 is: EQU Q2=I1.times.T2
Here, T1 is a time period when output signal K is high level and T2 is a time period when output signal K is low level, as shown in FIG. 8.
Thus, since the quantity of electric charge Q1 stored and the quantity of electric charge Q2 discharged are equal, we have: EQU (I2-I1).times.T1=I1.times.T2 EQU I1.times.(T1+T2)=I2.times.T1 EQU 1/(T1+T2)=I1/(I2.times.T1)
T1 is given by T1=1/fCLK from a frequency fCLK of pulse signal CLK of pulse signal generator 5, and so is constant.
If the input offset voltage of an operational amplifier OP1 of integrator 1 is zero, a minus input potential of operational amplifier OP1 is reference potential AG which is usually zero volts. From a minus reference potential VSS and a resistor R2, reference current I2 is fixed at I2=VSS/R2. From a resistor R1 and input voltage E of the voltage to frequency converter, input current I1 is obtained as I1=E/R1. As a result, an output frequency f of the voltage to frequency converter obtained at an output terminal Tout on substitution in the foregoing expression, is as follows: ##EQU1##
Thus, output frequency f has a value proportional to input voltage E.
Although, in the above explanation, it is assumed that the input offset voltage of operational amplifier OP1 is zero, usually the input offset voltage has a value V. In this case, minus input potential of operational amplifier OP1 is V. An output frequency fE of the voltage to frequency converter in this case is then obtained by the following expression: ##EQU2##
In this case, the linearity of input voltage E and output frequency fE of the voltage to frequency converter is poor. The relation between an linear error ERR and input voltage E when operational amplifier OP1 has an input offset voltage V is given by the following expression, and has the characteristic shown in FIG. 9. EQU ERR=(fE-f).times.100/f [%]